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INA284 current measurement

Other Parts Discussed in Thread: INA284, INA321, OPA333

I wanna to design a circuit to measurement the value of the Rload which is a resistive load.Here,I use the INA284 to measure the low current across the Rload and then use the voltage measurement circuit which deos not mentioned above to measure the voltage,Vload, acrossed the Rload.But AM3 which is 41nA does not equal to AM1 which is 35.5uA.We know that Rload=Vload/I.Here is my question: which current should I choose?AM3 or AM1.

My English is not good enough so if there have some grammatical mistakes.I wanna to say sorry.

Best regards,

Hugo

  • Hi hu shuanglu,

          What is the source of AM3 in your circuit? I guess if AM3 is increased to mA range, then AM3 and AM1 will become almost equal. Typical value of Input bias current for your INA284 is 25uA.

    But supply voltage that sources AM3 must ensure that common mode input voltage is not going to affect your output voltage.

  • Hi Deepak murali,

         AM3 is from a constant current source which ranges from 0 to 200uA.So the value of input bias current of the INA284 can not be ignored.By the way,the maxium of common mode input voltage is less than 20V.

         Is there a perfect solution even  not use the INA284 to measure the current across the Rload?

    Best regards,

    Hugo

  • Hi hu shuanglu,

      I guess using low bias current INA can be made used. Like INA321. Check ti.com for selecting better INA. And 20 V must be maximum common voltage. Usually output voltage is function of common voltage also. 

  • Hi Deepak murali,

        I have solved this problem by using an opa333 whose input bias current is only 70pA.In order to meet the requirement of  the common mode voltage,low side measurement is a nice choice.The simulation result  of my current measurement circuit  is good.And then I will design the PCB.

        Do you have any advice for my PCB?

    Best regards,

    Hugo

  • Hi hu shuanglu hu,

     

      Good that you have solved it. Am sorry. I don't make PCBs and not good enough to advice.